1. Field of the Invention
The present invention relates to a heat-sensitive recording material and especially a heat-sensitive recording material excelled in surface gloss, printing concentration, sticking resistance, resistance to water and solvent et cetera.
2. Description of the Prior Art
A heat-sensitive recording material having a dyestuff type heat-sensitive color-developing layer represented by combination of a leuco dye and a phenolic acidic substance is now being widely used in the field of thermal printers, facsimiles, measuring instruments recorders and, with its developing property, paper whiteness, applicability to various recording devices and economic feature highly appreciated, new uses are still being developed even now and, as a result, the requirement for improvement in quality of heat-sensitive recording material is being variegated and being further raised.
Conventional heat-sensitive recording materials made by applying heat-sensitive color-developing coating to a supporting member have defects of printed image area being erased or non-image area being developed when they are exposed to some of the known solvents, water, light, plasticizers et cetera. The same tendency is noted when they are stored for a long time, hence desired is improvement of storage stability of heat-sensitive recording material.
As a method of eliminating the aforementioned defects there has been developed a method of providing a protective coating on the heat-sensitive color-developing layer. As a binder for forming this protective coating has hitherto been known water-soluble high polymers such as polyvinyl alcohol, hydroxyethylcellulose, methylcellulose, carboxymethylcellulose, starches, caseins, polyacrylamide-type polymers, styrene-anhydrous maleic acid copolymer and polyacrylate and aqueous emulsions such as SBS latex (SBS is styrene-butadiene-styrene) but since protective coatings using some of the aforementioned coating compositions are not so good in sticking resistance, noise is apt to be caused during image formation, and in some cases the coating adheres to the thermal head, this often resulting in failure to obtain a proper image.
For eliminating the aforementioned defects and improving the sticking resistance there have been proposed methods of using in combination inorganic pigments, cellulose powder, microfine glass particles, colloidal silica, thermosetting resin, silicone type compounds et cetera (for example, Patent Publication No. 58-35874, Patent Publication No. 63-63397, Laid-open Patent Publication No.57-120489, Laid-open Patent Publication No. 60-18385, Laid-open Patent Publication No. 62-156990 and Laid-open Patent Publication No. 62-244693). By the use of the aforementioned additives it was indeed possible to improve the sticking resistance, but this caused such other problems as poor gloss and low printing concentration.